(A) Field of the Invention
The present invention relates to a structure for a resistance device and method of making the same, and in particular, to a thermistor device and method of making the same, which make use of a polymeric composite filled with a conductive filler and show resistance variations under different temperatures.
(B) Description of Related Art
Thermistor devices have already been widely used in many fields, such as temperature detection, security control, and temperature compensation. In the past, a thermistor device mainly utilizes a ceramic material, but ceramic material needs to be manufacture at a high temperature. In most cases, the temperature can be higher than 900xc2x0 C. Thus the energy consumption is enormous, and the process is also very complicated.
Later on, a thermistor device utilizing a polymeric substrate is developed. Because the manufacturing temperature of a thermistor device employing a polymeric substrate is under 300xc2x0 C., it can be easily manufactured and molded. The energy consumption is less, process is easier, and production cost is lower, so its application gets more and more popular as time goes by.
The temperature coefficient of the polymeric composite filled with a conductive filler will show different positive temperature coefficient resistance characteristics in accordance with different quantity of composite contained and different micro structures. This nature can be used to make a variety of resistance devices and positive temperature coefficient thermistor devices.
The Raychem Co. of U.S. utilizes the nature described above to produce a series of resetable polymeric positive temperature coefficient (PPTC) thermistor device (U.S. Pat. No. 4,237,441). When the temperature of the PPTC device reaches a certain switching temperature, the resistance of the PPTC device rises rapidly. Thus it can be applied to the design of over-current protection devices and temperature switch to devices. It can also be made into a Constant Wattage Element (CW type element, U.S. Pat. No. 4,304,987) that has a low sensitivity toward temperature variation. In this manner, it can be applied to the design of heaters.
But the polymeric thermistors of such kind are all positive temperature coefficient thermistors or devices that have low sensitivity toward temperature variation. The resistances either rise with the rising temperature or stay steady without changing with temperature variation. That is to say, the circuit design in actual circuit that applies a thermistor device is limited by the relations between temperature and resistance. For example, if we want to design a circuit, which is automatically activated when temperature reaches a certain level, an additional designed, complicated circuit has to be utilized instead of the traditional polymeric thermistor.
An object of the present invention is to provide a structure for a polymeric thermistor that has a negative temperature coefficient, so that the circuit design and application are not restricted to the traditional polymeric positive temperature coefficient thermistor. The application of the polymeric thermistor can be thus broadened.
Another object of the present invention is to provide a structure for a polymeric thermistor, wherein when it is put to use for the first time, the resistance is maintained in a relatively high status; but once it has been put to use at a high temperature, which means the temperature of the device has been risen to the glass-transition temperature or melting point of the polymeric substrate, the resistance would be relatively lowered down.
Yet another object of the present invention is to provide a manufacturing method of a structure for a polymeric thermistor, in which a simple-sheared process is used to change the microstructure of the conductive filler.
Still another object of the present invention is to provide a manufacturing method of a structure for a polymeric thermistor. The method manufactures polymeric thermistors filled with conductive filler which have different thermal-sensing natures. Thus, a new perspective of the possible application of the process is given.
To achieve the objects described above, the present invention provides a structure for a polymeric thermistor comprising: a polymeric composite filled with a conductive filler, the polymeric composite including a polymeric substrate; and conductive particles exist in the polymeric substrate, the conductive particles forming a discontinuous phase along a single direction. The polymeric composite has a characteristic of memorize shapes, and when it experiences a certain temperature (the certain temperature is the glass-transition temperature for amorphous thermoplastic materials or thermosetting materials, whereas it is the melting point for crystalline thermoplastic materials), the conductive particles that from a discontinuous phase along a single direction join to each other and become a conductive continuous phase. Thus, the mechanical stress of the polymeric thermistor is eliminated and the conductivity rises after temperature rose, so the polymeric thermistor can be a thermistor having a negative temperature coefficient, or in another case, when the polymeric thermistor is heated, the resistance of the polymeric thermistor can be lowered to a constant value. With the characteristics, the design and application of related circuits are not restricted to the traditional polymeric positive temperature coefficient thermistor and thus, the application of the polymeric thermistor is broadened.
Moreover, the method of manufacturing a structure for a polymeric thermistor provided by the present invention performs a cross-linking process to the polymeric substrate filled with conductive particles, so that the whole structure of polymeric composite filled with conductive particles is able to memorize shapes. Then, a simple-sheared process is performed to the polymeric composite for it to have a strain more than 1%, and thus changes the microstructure and electrical properties of the conductive filler.